1. Field of the Invention
The present invention relates to the fabrication of Josephson tunnel junctions, and more particularly to a process for trimming Josephson current by ion implantation into the junction.
2. Description of the Prior Art The trimming of Josephson current parameters has been taught in the prior art by various techniques. Tailoring Josephson current density (J.sub.1) by annealing is discussed in the publication "Annealing of Josephson Junction Devices" by S. Basavaiah et al, IBM Technical Disclosure Bulletin, Vol. 17, No. 11, April 1975, page 3488. In the publication "Rugged Josephson Elements and Arrays" by R. B. Laibowitz et al, IBM Technical Disclosure Bulletin, Vol. 17, No. 3, August 1974, page 826, the critical temperature (T.sub.c) and current (I.sub.c) are varied by annealing via heating through lasers or electron beams. Varying critical temperature T.sub.c by laser irradiation is also taught in the publication by N. Braslau et al, "Fabrication of Planar Josephson Junctions by Laser Irradiation", IBM Technical Disclosure Bulletin, Vol. 18, No. 11, April 1976, pages 3845, 3846.
The variation of critical temperature by ion bombardment damage is discussed in the E. P. Harris et al publication "Superconducting Device Fabrication by Energetic Ion Damage", IBM Technical Disclosure Bulletin, Vol. 17, No. 2, July 1974, page 604.
The use of ion implantation in Josephson circuits for other purposes are taught in the E. P. Harris et al publication, "Superconducting Microcircuitry for Layered Compounds Using Ion Implantation", IBM Technical Disclosure Bulletin, Vol. 17, No. 2, June 1974, pp. 257, 258 and the E. P. Harris et al publication "Applications of Ion Implantation to the Josephson Tunneling Technology", IBM Technical Disclosure Bulletin, Vol. 20, No. 6, November 1977, pages 2435, 2436.
U.S. Pat. No. 4,176,365 issued Nov. 27, 1979 to Kroger, entitled JOSEPHSON TUNNEL JUNCTION DEVICE WITH HYDROGENATED AMORPHOUS SILICON GERMANIUM OR SILICON-GERMANIUM ALLOY TUNNELING BARRIER describes Josephson tunnel device having an unusually high critical current density wherein dopants provide a further increase in the critical current density. The barrier is deposited by rf-sputtering in an atmosphere containing hydrogen.
U.S. Pat. No. 3,906,231 issued Sept. 16, 1975 to Fletcher et al entitled DOPED JOSEPHSON TUNNELING JUNCTION FOR USE IN A SENSITIVE IR DETECTOR describes a Josephson device with a modified tunnel barrier. The tunnel barrier located between a pair of electrodes includes a molecular species which is capable of coupling incident radiation of a spectrum characteristic of the molecular species into the tunnel barrier. The coupled radiation modulates the known Josephson characteristics of the superconducting device. As a result, a superconductive tunneling device can be tuned or made sensitive to a particular radiation associated with the dopant molecular species.
Ion implantation has been used in the prior art to modify other electrical properties of other type devices: T. W. Sigmon and R. Swanson, Solid State Electronics, 16, 1217 (1973) and P. D. Taylor and D. V. Morgan, Solid State Electronics, 19, 473, 481, 935 (1976).
However, the application of ion implantation for the purposes of directly trimming the zero voltage Josephson current in a completed tunnel junction is quite distinct from the teachings of such references.
Other references which relate to the general technical area of the present invention include the following:
IBM Technical Disclosure Bulletins: PA0 U.S. Pat. Nos.
Vol. 17, No. 4, September 1974, page 1204, PA2 Vol. 17, No. 4, September 1974, page 1207, PA2 Vol. 17, No. 6, November 1974, page 1827. PA2 3,755,092 by Antula on Aug. 28, 1973, PA2 4,299,679 by Suzuki on Nov. 10, 1981.